The present invention relates to a process for the production of isocyanates or mixtures of isocyanates which contain no appreciable amount of color-causing materials, the isocyanates produced by this process and a process for the production of light-colored foams from these polyisocyanates.
Improved color of rigid polyurethane foams is one of the goals sought by many working in polyurethane chemistry. Pale yellow to white is the desired color. In addition to a better visual impression, light-colored foams indicate that the raw materials from which they were produced had a high degree of purity.
If a yellowish brown or grey starting material is used to produce a foam, undesirable streaks occur at the surface of the rigid foam. These streaks are due to bubbling up during the foaming process. The exact composition of the materials causing the color in polyurethane foams or of the color-causing components in the polymer has not yet been established.
The prior art teaches that the elimination of color is one of the problems addressed during the preparation of isocyanates. A number of methods for improving the color of polyurethane foams have been described in the literature.
For example, GB-A 2,207,671 discloses the use of dried magnesium silicates to remove traces of "coloring impurities" in diisocyanatodiphenylmethane (MDI) at around 190.degree. C.
U.S. Pat. No. 4,465,639 teaches that the "dark colored material" formed during separation of the solvent in the preparation of diisocyanatodiphenylmethane, is reduced by the addition of small quantities of water.
U.S. Pat. No. 4,876,380 describes an extraction method in which polyisocyanates are separated into two fractions by using methylene dichloride and pentane. One of the fractions is a purified fraction having improved color. The color of the second fraction is not critical. Purification of polyisocyanates by extraction is also described in EP-A 0,133,538.
In EP-A 0,446,781, a method for the improvement of polyurethane foam is described. In this method, diaminodiphenylmethane (MDA) is subjected to a catalytic hydrogenation in the presence of hydrogenation catalysts and a specific water content. All of the catalysts known to be useful in catalytic hydrogenation processes are taught to be suitable hydrogenation catalysts. One of the disadvantages of this process is that during the hydrogenation, by-products which remain in the MDA are formed. Under unfavorable phosgenation conditions, these by-products still form color causing impurities in the MDI. Moreover, it is known that the formation of strongly colored compounds in amounts in the ppm range is not excluded by the phosgenation reaction.
EP-A 561,225 discloses that a distinct improvement in the color of isocyanates is achieved without adversely affecting the other important properties of the isocyanates through a simple hydrogen treatment in the presence of catalysts at 3 to 150 bar and at 100.degree. to 180.degree. C. However, the activity and loading capacity of the catalysts used and the improvement in color achieved are less than desirable.